I know that if I set in the vhost for each website the specific port like:
mysiteonvps1.com:81
mysiteonvps2.com:82
...etc.. I can have different website on different machines on the same Ip, but I want to use only the 80 port.

If I have correctly understand,
I will set a "master" vps server that will be visible from internet and this will pass the wan requests using the proxy to the two Lan vps.

To be clear.. to do this, I only need to set apache2 with the mod_proxy, on a clean debian without ispconfig, .. and pass the wan url request to the lan server; and on the lan server set the virtual host on each vps (i'm using ispconfig with apache2 and ngin respective)